Gary, take a look at CT1. This is designed for the building trade rather than the rip-off boat trade, used on the London Eye. I use it for everything on deck now. It's cheaper than Sikaflex etc and, claim its manufacturers, not as dodgy for the environment as everything else. It cleans off easily, sets whether rained on or not, and you can scrub it off your fingernails with soap. I recaulked
Paradox's deck with it and bedded down fittings. So far it's lasted well and looks good. Here's the
link. Best place I found to get it from was
here. I haven't used it underwater yet, but I think I would - local marina engineers here in Brighton seem to use it for everything.
Edit - Oops. just noticed you're in Oz. Oh well, check the building trade first.